"Many of the applications we envision for nanoparticles, such as optical coatings and photovoltaic and magnetic storage devices, require planar geometry," said Sunita Srivastava, a Stony Brook University postdoctoral researcher and the lead author on the paper. Other groups of scientists have assembled such planes of nanoparticles, essentially floating them on a liquid surface, but these single-layer arrays have all been static, she explained. "Using DNA linker molecules gives us a way to control the interactions between the nanoparticles."
As described in the paper, the scientists demonstrated their ability to achieve differently structured monolayers, from a viscous fluid-like array to a more tightly woven cross-linked elastic meshand switch between those different statesby varying the strength of the pairing between complementary DNA strands and adjusting other variables, including the electrostatic charge on the liquid assembly surface and the concentration of salt.
When the surface they used, a lipid, has a strong positive charge it attracts the negatively charged DNA strands that coat the nanoparticles. That electrostatic attraction and the repulsion between the negatively charged DNA molecules surrounding adjacent nanoparticles overpower the attractive force between complementary DNA bases. As a result, the particles form a rather loosely arrayed free-floating viscous monolayer. Adding salt changes the interactions and overcomes the repulsion between like-charged DNA strands, allowing the base pairs to match up and link the nanoparticles together more closely, first forming string-like arrays, and with more salt, a more solid yet elastic mesh-like layer.
"The mechanism of this phase transition is not obvious," said Gang. "It cannot be understood from the repulsion-attraction interactions alone. With the help
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DOE/Brookhaven National Laboratory